You should try offering him a range of different foods: moist kitten food, dry kitten kibble, soaked kitten kibble (so that it is mushy). If he is still not eating this, you should take him to your vet to have him checked in case there is a medical reason for this unusual behavior.

The mention was as follows: “Small amounts of kitten milk replacer or even cow’s milk may be added to the food gruel: this may make the taste more appealing to kittens familiar with the taste of milk” – so the point is that this is not suggesting that cow’s milk is a complete or appropriate food, but rather that it will give the food gruel some of the taste of milk, which will make it more palatable to kittens that are used to the taste of milk. There is nothing fundamentally wrong with cow’s milk for cats: it contains lactose, which kittens can digest, but which adult cats lose the ability to digest. That’s why it is normally recommended not to give cats milk.
I hope this helps
Pete